It has been known in the past to place an exhaust sensor in an exhaust passage of an internal combustion engine to detect a specific component in exhaust gas (for example, see Japanese Patent Publication No. 2009-299538A). Such an exhaust sensor is provided with a sensor element, a sensor cover covering the sensor element, and a heater heating the sensor element. The exhaust sensor is arranged in an exhaust pipe so as to be exposed to the exhaust gas. Part of the exhaust gas flows into the sensor element.
When the temperature of the exhaust pipe is the dew point of water or less, the water vapor in the exhaust gas condenses and condensed water is produced. Part of the condensed water enters into the sensor cover of the exhaust sensor. When there is condensed water inside the sensor cover, if exhaust gas flows into the sensor cover, drops of the condensed water will strike the sensor element together with the exhaust gas. If the sensor element does not have water repellency, the drops of water striking the sensor element will penetrate into the sensor element. Further, if the temperature of the sensor element is high due to heating by the heater, the water drops penetrating into the sensor element will evaporate inside the sensor element. As a result, the sensor element will be given thermal shock and the sensor element will sometimes crack.
Regarding this problem, Japanese Patent Publication No. 2009-299538A describes to prevent a cracked element of the exhaust sensor by raising the temperature of the exhaust system by driving a crankshaft to rotate by a motoring means before starting up the internal combustion engine and making the heater start after the ambient temperature of the exhaust sensor reaches the dew point.